How can I make the efficent for loop in Python?

Question

I am writing the csv file in python and there are four levels of nested objects. like

I need to show the csv like this

StudentName, StudentCla

Answer 1

You should Initialize s.subjects and subject.books with empty lists. That way you won't get an error when trying to iterate them here or on other places in your code.

Answer 2

Initialize your student.subjects and subject.books with [], and then it'll work. Just as what @user714965 said.

or, use list-comprehension:

for s in [stu if stu.has_key(subjects) for stu in students]:
    for subject in [subj if subj.has_key(books) for subj in s.subjects]:
         lists= [(s.name, s.class, subject.name, book.name) for book in subject.books]
              print r'\n'.join(r','join(lists))

or

if your loop is not very deep,try something like this:

fn= s.has_key('subjects') and (lambda s: /do sth. with s/ ) or (lambda s: /do sth. else with s/)

in your loop, use the fn like this,

[fn(s) for s in students ]

Answer 3

Assuming s.subjects is None or some other False value when there are no subjects, and likewise for books

for s in students:
    for subject in s.subjects or []:
        for book in subject.books or []:
            writer.writerow(s.name, s.class, subject.name, book.name)

More generally, you can write

for s in students:
    for subject in s.subjects if <condition> else []:
        for book in subject.books if <condition> else []:
            writer.writerow(s.name, s.class, subject.name, book.name) 

Where <condition> is whatever expression makes sense